Most pathways for bacterial chemotaxis involve methylation of the sensory transducer. Studies in this laboratory are concerned with methylation-independent pathways in Salmonella typhimurium and Escherichia coli for chemotaxis to oxygen (aerotaxis) and to sugars transported by the phosphotransferase system (PTS). The methylation-independent and the methylation-dependent pathways converge before the switch -that controls the direction of rotation of the flagellar motors. The goal is to describe in molecular detail the signal processing mechanisms in these behaviors. The convergence of the PTS system and the main chemotaxis pathway will be investigated using genetic analysis and reconstitution of the system with purified proteins. A sensitive spectro-photometric assay for chemotaxis will be developed for use in the reconstitution studies. Mutants with a defect in aerotaxis will be selected and characterized, the mutation mapped and the gene cloned. Components interacting with the aerotaxis gene product will be identified by allele-specific suppression analysis. The proposed mechanism will be confirmed by reconstitution with purified components. E. coli and S. typhimurium are repelled by high concentrations of oxygen. The physiology of the response will be investigated, including repression or induction, a requirement for oxygen consumption or redox sensing and the additivity of the oxygen-avoidance response and chemotactic responses. Genetic and biochemical investigations similar to those described above will be used to determine components of the avoidance response and the site of convergence with the chemotaxis pathway. A possible role of reactive oxygen species or AppppA in mediating the response will be investigated. Aerotaxis in Halobacterium halobium is the only aerotaxis that is methylation-dependent and we will further investigate the mechanism of this response.